[1.8.x] Fixed #23940 -- Allowed model fields to be named `exact`.

An explicit `__exact` lookup in the related managers filters
was interpreted as a reference to a foreign `exact` field.

Thanks to Trac alias zhiyajun11 for the report, Josh for the investigation,
Loïc for the test name and Tim for the review.

Backport of eb4cdfbd from master